A map of all the identities that are associated with this user's account. The keys of the map can be any of the following: email, phone,
google.com, facebook.com, github.com, twitter.com. The values of the map are lists of unique identifiers for each identitity provider associated with the account. For example,
request.auth.token.firebase.identities["google.com"][0] contains the first Google user ID associated with the account.

firebase.sign_in_provider

The sign-in provider used to obtain this token. Can be one of the following strings: custom, password, phone, anonymous,
google.com, facebook.com, github.com, twitter.com.